Experimental observation of the improvement in MTF from backthinning a CMOS direct electron detector

The advantages of backthinning monolithic active pixel sensors (MAPS) based on complementary metal oxide semiconductor (CMOS) direct electron detectors for electron microscopy have been discussed previously; they include better spatial resolution (modulation transfer function or MTF) and efficiency at all spatial frequencies (detective quantum efficiency or DQE). It was suggested that a 'thin' CMOS detector would have the most outstanding properties [1-3] because of a reduction in the proportion of backscattered electrons. In this paper we show, theoretically (using Monte Carlo simulations of electron trajectories) and experimentally that this is indeed the case. The modulation transfer functions of prototype backthinned CMOS direct electron detectors have been measured at 300keV. At zero spatial frequency, in non-backthinned 700-mum-thick detectors, the backscattered component makes up over 40% of the total signal but, by backthinning to 100, 50 or 35mum, this can be reduced to 25%, 15% and 10%, respectively. For the 35mum backthinned detector, this reduction in backscatter increases the MTF by 40% for spatial frequencies between 0.1 and 1.0 Nyquist. As discussed in the main text, reducing backscattering in backthinned detectors should also improve DQE.

Detective quantum efficiency of electron area detectors in electron microscopy

Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.

Encouraging attendance at outpatient appointments: can we do more?

BACKGROUND AND AIMS

Non-attendance at new outpatient appointments is a challenge for health services. Patients with treatable morbidity may fail to receive care from which they would benefit, and scarce resources are wasted. Therefore, a literature review was undertaken to assess the effectiveness of appointment reminders as a means of increasing attendance and reducing 'Did not Attend' rates at new outpatient appointments.

METHODS

Electronic databases were interrogated using a combination of MeSH terms and truncated keywords. In addition, the reference lists of papers were studied to identify additional relevant material.

RESULTS

Telephone and postal reminders appear effective, particularly if received within a day of scheduled appointments. Postal reminders offering a reward for attendance, threatening sanctions for non-attendance, and providing information about the clinic appear more effective than standard reminders. No studies assessing the impact of measures including e-mail or text messaging were identified. Many studies were conducted in psychiatric settings outwith the United Kingdom raising issues of generalisability. In addition, no assessment of how initiatives impact on inequalities in service access were found.

CONCLUSIONS

There is scope to increase attendance and reduce non-attendance at outpatient appointments but initiatives should be piloted in conjunction with robust evaluative frameworks.

Reduction in young male suicide in Scotland

Background

Rates of suicide and undetermined death increased rapidly in Scotland in the 1980's and 1990's. The largest increases were in men, with a marked increase in rates in younger age groups. This was associated with an increase in hanging as a method of suicide. National suicide prevention work has identified young men as a priority group. Routinely collected national information suggested a decrease in suicide rates in younger men at the beginning of the 21^st century. This study tested whether this was a significant change in trend, and whether it was associated with any change in hanging rates in young men.

Methods

Joinpoint regression was used to estimate annual percentage changes in age-specific rates of suicide and undetermined intent death, and to identify times when the trends changed significantly. Rates of deaths by method in 15 – 29 year old males and females were also examined to assess whether there had been any significant changes in method use in this age group.

Results

There was a 42% reduction in rates in 15 – 29 year old men, from 42.5/100,000 in 2000 to 24.5/100,000 in 2004. A joinpoint analysis confirmed that this was a significant change. There was also a significant change in trend in hanging in men in this age group, with a reduction in rates after 2000. No other male age group showed a significant change in trend over the period 1980 – 2004. There was a smaller reduction in suicide rates in women in the 15 – 29 year old age group, with a reduction in hanging from 2002.

Conclusion

There has been a reduction in suicide rates in men aged 15 – 29 years, and this is associated with a significant reduction in deaths by hanging in this age group. It is not clear whether this is related to a change in method preference, or an overall reduction in suicidal behaviour, and review of self-harm data will be required to investigate this further.

Electronic detectors for electron microscopy

Due to the increasing popularity of electron cryo-microscopy (cryoEM) in the structural analysis of large biological molecules and macro-molecular complexes and the need for simple, rapid and efficient readout, there is a persuasive need for improved detectors. Commercial detectors, based on phosphor/fibre optics-coupled CCDs, provide adequate performance for many applications, including electron diffraction. However, due to intrinsic light scattering within the phosphor, spatial resolution is limited. Careful measurements suggest that CCDs have superior performance at lower resolution while all agree that film is still superior at higher resolution. Consequently, new detectors are needed based on more direct detection, thus avoiding the intermediate light conversion step required for CCDs. Two types of direct detectors are discussed in this review. First, there are detectors based on hybrid technology employing a separate pixellated sensor and readout electronics connected with bump bonds-hybrid pixel detectors (HPDs). Second, there are detectors, which are monolithic in that sensor and readout are all in one plane (monolithic active pixel sensor, MAPS). Our discussion is centred on the main parameters of interest to cryoEM users, viz. detective quantum efficiency (DQE), resolution or modulation transfer function (MTF), robustness against radiation damage, speed of readout, signal-to-noise ratio (SNR) and the number of independent pixels available for a given detector.

Electron imaging with Medipix2 hybrid pixel detector

The electron imaging performance of Medipix2 is described. Medipix2 is a hybrid pixel detector composed of two layers. It has a sensor layer and a layer of readout electronics, in which each 55 microm x 55 microm pixel has upper and lower energy discrimination and MHz rate counting. The sensor layer consists of a 300 microm slab of pixellated monolithic silicon and this is bonded to the readout chip. Experimental measurement of the detective quantum efficiency, DQE(0) at 120 keV shows that it can reach approximately 85% independent of electron exposure, since the detector has zero noise, and the DQE(Nyquist) can reach approximately 35% of that expected for a perfect detector (4/pi(2)). Experimental measurement of the modulation transfer function (MTF) at Nyquist resolution for 120 keV electrons using a 60 keV lower energy threshold, yields a value that is 50% of that expected for a perfect detector (2/pi). Finally, Monte Carlo simulations of electron tracks and energy deposited in adjacent pixels have been performed and used to calculate expected values for the MTF and DQE as a function of the threshold energy. The good agreement between theory and experiment allows suggestions for further improvements to be made with confidence. The present detector is already very useful for experiments that require a high DQE at very low doses.

Digitisation of electron microscope films: Six useful tests applied to three film scanners

A series of simple tests have been used to measure the performance of flat-bed film scanners suitable for digitisation of electron micrographs. Two of the film scanners evaluated are commercially available and one has been constructed in the laboratory paying special attention to the needs of the electron microscopist. The tests may be useful for others.

Changes in Scottish suicide rates during the Second World War

Background

It is believed that total reported suicide rates tend to decrease during wartime. However, analysis of suicide rates during recent conflicts suggests a more complex picture, with increases in some age groups and changes in method choice. As few age and gender specific analyses of more distant conflicts have been conducted, it is not clear if these findings reflect a change in the epidemiology of suicide in wartime. Therefore, we examined suicide rates in Scotland before, during and after the Second World War to see if similar features were present.

Methods

Data on deaths in Scotland recorded as suicide during the period 1931 – 1952, and population estimates for each of these years, were obtained from the General Register Office for Scotland. Using computer spreadsheets, suicide rates by gender, age and method were calculated. Forward stepwise logistic regression was used to assess the effect of gender, war and year on suicide rates using SAS V8.2.

Results

The all-age suicide rate among both men and women declined during the period studied. However, when this long-term decline is taken into account, the likelihood of suicide during the Second World War was higher than during both the pre-War and post-War periods. Suicide rates among men aged 15–24 years rose during the Second World War, peaking at 148 per million (41 deaths) during 1942 before declining to 39 per million (10 deaths) by 1945, while the rate among men aged 25–34 years reached 199 per million (43 deaths) during 1943 before falling to 66 per million (23 deaths) by 1946. This was accompanied by an increase in male suicides attributable to firearms and explosives during the War years which decreased following its conclusion.

Conclusion

All age male and female suicide rates decreased in Scotland during World War II. However, once the general background decrease in suicide rates over the whole period is accounted for, the likelihood of suicide among the entire Scottish population during the Second World War was elevated. The overall decrease in suicide rates concealed large increases in younger male age groups during the War years, and an increase in male suicides recorded as due to the use of firearms. We conclude that the effects of war on younger people, reported in recent conflicts in Central Europe, were also seen in Scotland during the Second World War. The results support the findings of studies of recent conflicts which have found a heterogeneous picture with respect to age specific suicide rates during wartime.

Prospective neurocognitive effects and quality of life (QOL) in patients with multiple brain metastases receiving whole brain radiation (WBRT) ± thalidomide on radiation therapy oncology group (RTOG) trial 0118

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Background: RTOG 0118 was a randomized trial of WBRT (37.5 Gy/15 fractions) ± thalidomide in patients (pts) with brain metastases, which showed no effect on survival (Knisely et al, ASCO, 2005). This analysis examined the relationship between neurocognitive progression (NCP) and QOL in patients on both arms. Methods: NCP was assessed with the Folstein Mini-Mental Status Exam (MMSE) and QOL with the Spitzer QOL Index (SQLI). Pts with MMSE scores below age/education adjusted cutoff levels were considered to have NCP. T-test and Chi-Square statistics were used to determine significant differences in NCP and QOL between the two treatment arms over time. Results: Of 176 pts, 88 and 92% had SQLI and MMSE scores at baseline (43% were below the MMSE cutoff). Baseline SQLI (but not MMSE) was a significant predictor for OS (p=0.034). From baseline, SQLI and MMSE compliance rates were 74 and 60% at 1–2 months (mo), 37 and 29% at 4 mo, and 21 and 19% at 6 months, respectively. While QOL was stable, those who failed to meet MMSE cutoffs increased by 60% by 6 mo, on both study arms. Although a weak correlation (r=.23) existed at baseline between QOL and MMSE for both the WBRT (0.04) and WBRT/thalidomide (0.058) arms, there was no correlation beyond baseline. Conclusions: Baseline QOL is a significant predictor of OS. While QOL remained essentially stable, the degree of neurocognitive decline (60%) over time is concerning. These findings suggest the importance of these outcome measures in pts with brain metastases and the need for interventions to conserve cognitive status. RTOG is developing a trial to study the role of preventive strategies for NCP in brain metastases.

No significant financial relationships to disclose.

A note on testing for intervention effects on binary responses

OBJECTIVES

In some circumstances controlled trials are not feasible and treatments can only be evaluated using clinical databases. Here we consider the situation where treatment is introduced at a particular calendar time and can only be evaluated by comparison with historical controls. In these circumstances Heuer and Abel recommended using change-point methods to search for change in characteristics over the whole study period rather than simply comparing treated and untreated patients. Their recommendation is to only conclude that the intervention had an effect if a change-point could be demonstrated close in time to the introduction of the new treatment. This reduces the risk of false positives caused by confounding changes in population characteristics or changes in patient management. For binary data we develop a method that follows their philosophy and apply it to an observational study in the treatment of pin sites after orthopaedic surgery.

METHODS

Tests for change in binomial probabilities based on Brownian bridge and Hansen's approximation for maximally selected chi(2) statistics are compared to an exact test by Worsley. The approximate method is generalized to logistic regression models allowing for covariates.

RESULTS

The agreement of the exact and approximate method is good for sample sizes of 100 or more. The actual test size of the Hansen approximate test allowing for covariates is close to the nominal level, whereas the Brownian bridge approximation is slightly conservative. The change in pin site treatment significantly reduces the risk of infection for both adults and children.

CONCLUSIONS

We consider the Hansen approximation to provide a very good and very simple method for obtaining the p-value when testing for a change in binary data event probabilities, with or without covariates.

Individual survival time prediction using statistical models

Doctors' survival predictions for terminally ill patients have been shown to be inaccurate and there has been an argument for less guesswork and more use of carefully constructed statistical indices. As statisticians, the authors are less confident in the predictive value of statistical models and indices for individual survival times. This paper discusses and illustrates a variety of measures which can be used to summarise predictive information available from a statistical model. The authors argue that models and statistical indices can be useful at the group or population level, but that human survival is so uncertain that even the best statistical analysis cannot provide single-number predictions of real use for individual patients.

General practitioners' concerns about childhood immunisation and suggestions for improving professional support and vaccine uptake

In recent years childhood vaccination has been vigorously debated. Professional and parental confidence in the measles, mumps and rubella (MMR) vaccine in particular has been shaken, as reflected by its decreased uptake. A semi-structured postal questionnaire survey of general practitioners (GPs) working in the Highland region of Scotland was undertaken to assess their views on vaccination issues. A 73% response rate was achieved. Of respondents 28% expressed concerns about the possible side effects of MMR. Of GPs 98% thought it was 'likely' that the benefits of the DTP-Hib, polio and men C vaccines outweighed the possible risks. For the first and second doses of MMR 92% and 86%, respectively, believed this to be true. Approximately three quarters of GPs described themselves 'very confident' in discussing the DTP-Hib vaccine with parents, compared with 57% for MMR. Some respondents were misinformed regarding the adverse events associated with the MMR vaccine. There is professional concern about MMR. Initiatives to support GPs in promoting vaccination in order to improve immunisation rates are discussed.

Clinical effectiveness and cost-effectiveness of clopidogrel and modified-release dipyridamole in the secondary prevention of occlusive vascular events: a systematic review and economic evaluation

OBJECTIVES

To examine the clinical effectiveness and cost-effectiveness of two alternative antiplatelet agents, clopidogrel and modified-release (MR)-dipyridamole, relative to prophylactic doses of aspirin for the secondary prevention of occlusive vascular events.

DATA SOURCES

Electronic databases.

REVIEW METHODS

A total of 2906 titles and abstracts were rigorously screened and 441 studies were assessed in detail. Two RCTs were identified. For the assessment of cost-effectiveness, eight reviews were identified. The results were presented in structured tables and as a narrative summary. No additional clinical effectiveness data were presented in either of two company submissions. All economic evaluations (including accompanying models) included in the company submissions were assessed. Following this analysis, if the existing models (company or published) were not sufficient, a de novo model or modified versions of the models were developed.

RESULTS

In the CAPRIE trial the point estimate for the primary outcome, i.e. ischaemic stroke, myocardial infarction (MI) or vascular death, favoured clopidogrel over aspirin, but the boundaries of the confidence intervals raise the possibility that clopidogrel is not more beneficial than aspirin. In terms of the secondary outcomes reported, there was a non-significant trend in favour of clopidogrel over aspirin but the boundaries of the confidence intervals on the relative risks all crossed unity. There was no difference in the number of patients ever reporting any bleeding disorder in the clopidogrel group compared with the aspirin group. The incidences of rash and diarrhoea were statistically significantly higher in the clopidogrel group than the aspirin group. Patients in the aspirin group had a higher incidence of indigestion/nausea/vomiting than patients in the clopidogrel group. Haematological adverse events were rare in both the clopidogrel and aspirin groups. No cases of thrombotic thrombocytopenic purpura were reported in either group. Treatment with MR-dipyridamole alone did not significantly reduce the risk of any of the primary outcomes reported in ESPS-2 compared with treatment with aspirin. ASA-MR-dipyridamole was significantly more effective than aspirin alone in patients with stroke or transient ischaemic attacks (TIAs) at reducing the outcome of stroke and marginally more effective at reducing stroke and/or death. Treatment with ASA-MR-dipyridamole did not statistically significantly reduce the risk of death compared to treatment with aspirin. The number of strokes was statistically significantly reduced in the ASA-MR-dipyridamole group compared with the MR-dipyridamole group. In terms of the other primary outcomes, stroke and/or death and death, the results favoured treatment with ASA-MR-dipyridamole but the findings were not statistically significant. There was no difference in the number of bleeding complications between the ASA-MR-dipyridamole and aspirin groups. The incidence of bleeding complications was significantly lower in the MR-dipyridamole treatment group. More patients in the MR-dipyridamole treatment groups experienced headaches compared to patients receiving treatment with aspirin alone. The York model assessed the cost-effectiveness of differing combinations of treatment strategies in four patient subgroups, under a number of different scenarios. The results of the model were sensitive to the assumptions made in the alternative scenarios, in particular the impact of therapy on non-vascular deaths.

CONCLUSIONS

Clopidogrel was marginally more effective than aspirin at reducing the risk of ischaemic stroke, MI or vascular death in patients with atherosclerotic vascular disease, however, it did not statistically significantly reduce the risk of vascular death or death from any cause compared with aspirin. There was no statistically significant difference in the number of bleeding complications experienced in the clopidogrel and aspirin groups. MR-dipyridamole in combination with aspirin was superior to aspirin alone at reducing the risk of stroke and marginally more effective at reducing the risk of stroke and/or death. Compared with treatment with MR-dipyridamole alone, MR-dipyridamole in combination with aspirin significantly reduced the risk of stroke. Treatment with MR-dipyridamole in combination with aspirin did not statistically significantly reduce the risk of death compared with aspirin. Compared with treatment with MR-dipyridamole alone, bleeding complications were statistically significantly higher in patients treated with aspirin and MR-dipyridamole in combination with aspirin. Due to the assumptions that have to be made, no conclusions could be drawn about the relative effectiveness of MR-dipyridamole, alone or in combination with aspirin, and clopidogrel from the adjusted indirect comparison. The following would apply for a cost of up to GBP20,000-40,000 per additional quality-adjusted life-year. For the stroke and TIA subgroups, ASA-MR-dipyridamole would be the most cost-effective therapy given a 2-year treatment duration as long as all patients were not left disabled by their initial (qualifying) stroke. For a lifetime treatment duration, ASA-MR-dipyridamole would be considered more cost-effective than aspirin as long as treatment effects on non-vascular deaths are not considered and all patients were not left disabled by their initial stroke. In patients left disabled by their initial stroke, aspirin is the most cost-effective therapy. Clopidogrel and MR-dipyridamole alone would not be considered cost-effective under any scenario. For the MI and peripheral arterial disease subgroups, clopidogrel would be considered cost-effective for a treatment duration of 2 years. For a lifetime treatment duration, clopidogrel would be considered more cost-effective than aspirin as long as treatment effects on non-vascular deaths are not considered. It is suggested that the combination of clopidogrel and aspirin should be evaluated for the secondary prevention of occlusive vascular events. Also randomised, direct comparisons of clopidogrel and MR-dipyridamole in combination with aspirin are required to inform the treatment of patients with a history of stroke and TIA, plus trials that compare treatment with clopidogrel and MR-dipyridamole for the secondary prevention of vascular events in patients who demonstrate a genuine intolerance to aspirin.